Lightning is the most spectacular element of a thunderstorm. In fact it is how thunderstorms got their name. Wait a minute, what does thunder have to do with lightning? Well, lightning causes thunder.
Lightning is a discharge of electricity. A single stroke of lightning can heat the air around it to 30,000°C (54,000°F)! This extreme heating causes the air to expand explosively fast. The expansion creates a shock wave that turns into a booming sound wave, known as thunder.
What's Happening Within the Cloud?
As ice crystals high within a thunderstorm cloud flow up and down in the turbulent air, they crash into each other.
Small negatively charged particles called electrons are knocked off some ice and added to other ice as they crash past each other. This separates the positive (+) and negative (-) charges of the cloud.
The top of the cloud becomes positively charged while the base of the cloud becomes negatively charged.
How is a Lightning Bolt Formed?
Because opposites attract, the negative charge at the bottom of the storm cloud wants to link up with the ground’s positive charge. Once the negative charge at the bottom of the cloud gets large enough, a flow of negative charge called a stepped leader rushes toward the Earth.
The positive charges at the ground are attracted to the stepped leader, so positive charge flows upward from the ground. When the stepped leader and the positive charge meet, a strong electric current carries positive charge up into the cloud. This electric current is known as the return stroke.
We see it as the bright flash of a lightning bolt.
Thunder and lightning occur at roughly the same time although you see the flash of lightning before you hear the thunder. This is because light travels much faster than sound.
What Gives Lightning its Zap?
The accumulation of electric charges has to be great enough to overcome the insulating properties of air. When this happens, a stream of negative charges pours down towards a high point where positive charges have clustered due to the pull of the thunderhead.
The connection is made and the protons rush up to meet the electrons. It is at that point that we see lightning and hear thunder. A bolt of lightning heats the air along its path causing it to expand rapidly. Thunder is the sound caused by rapidly expanding air.
Zap! You just touched a metal doorknob after shuffling your rubber-soled feet across the carpet. Yipes! You've been struck by lightning! Well, not really, but it's the same idea.
Your rubber-soled shoes picked up stray electrons from the carpet. Those electrons built up on your shoes giving them a static charge. (Static means not moving.
) Static charges are always “looking” for the first opportunity to “escape,” or discharge.
Your contact with a metal doorknob—or car handle or anything that conducts electricity—presents that opportunity and the excess electrons jump at the chance.
What causes lightning?
So, do thunderclouds have rubber shoes? Not exactly, but there is a lot of shuffling going on inside the cloud.
Click image for animation.
Credit: John Jensenius
Lightning begins as static charges in a rain cloud. Winds inside the cloud are very turbulent.
Water droplets in the bottom part of the cloud are caught in the updrafts and lifted to great heights where the much colder atmosphere freezes them.
Meanwhile, downdrafts in the cloud push ice and hail down from the top of the cloud. Where the ice going down meets the water coming up, electrons are stripped off.
It's a little more complicated than that, but what results is a cloud with a negatively charged bottom and a positively charged top. These electrical fields become incredibly strong, with the atmosphere acting as an insulator between them in the cloud.
When the strength of the charge overpowers the insulating properties of the atmosphere, Z-Z-Z-ZAP! Lightning happens.
How does the lightning “know” where to discharge—or strike?
The electric field “looks” for a doorknob. Sort of. It looks for the closest and easiest path to release its charge. Often lightning occurs between clouds or inside a cloud.
But the lightning we usually care about most is the lightning that goes from clouds to ground—because that's us!
What causes lightning?
A lightning storm striking down in a rural area. Credit: noaanews.noaa.gov
Thunder and lightning. When it comes to the forces of nature, few other things have inspired as much fear, reverence, or fascination – not to mention legends, mythos, and religious representations. As with all things in the natural world, what was originally seen as a act by the Gods (or other supernatural causes) has since come to be recognized as a natural phenomena.
But despite all that human beings have learned over the centuries, a degree of mystery remains when it comes to lightning. Experiments have been conducted since the time of Benjamin Franklin; however, we are still heavily reliant on theories as to how lighting behaves.
By definition, lightning is a sudden electrostatic discharge during an electrical storm. This discharge allows charged regions in the atmosphere to temporarily equalize themselves, when they strike an object on the ground. Although lightning is always accompanied by the sound of thunder, distant lightning may be seen but be too far away for the thunder to be heard.
Lightning can take one of three forms, which are defined by what is at the “end” of the branch channel (i.e. lightning bolt).
For example, there is intra-cloud lighting (IC), which takes place between electrically charged regions of a cloud; cloud-to-cloud (CC) lighting, where it occurs between one functional thundercloud and another; and cloud-to-ground (CG) lightning, which primarily originates in the thundercloud and terminates on an Earth surface (but may also occur in the reverse direction).
Intra-cloud lightning most commonly occurs between the upper (or “anvil”) portion and lower reaches of a given thunderstorm. In such instances, the observer may see only a flash of light without hearing any thunder. The term “heat-lightning” is often applied here, due to the association between locally experienced warmth and the distant lightning flashes.
In the case of cloud-to-cloud lightning, the charge typically originates from beneath or within the anvil and scrambles through the upper cloud layers of a thunderstorm, normally generating a lightning bolt with multiple branches.
Cloud-to-ground (CG) is the best known type of lightning, though it is the third-most common – accounting for approximately 25% cases worldwide. In this case, the lightning takes the form of a discharge between a thundercloud and the ground, and is usually negative in polarity and initiated by a stepped branch moving down from the cloud.
Is Lightning Triggered by Cosmic Rays?
Lightning is a natural electrical discharge—but scientists are still scratching their heads trying to figure out what triggers it. Renowned Russian physicist Alexandr Gurevich tells Katia Moskvitch about his theory, which really is out of this world
What don’t we know about lightning?
The main problem is that we don’t know how a thundercloud gets the spark needed to initiate a lightning bolt. The biggest mystery is that the electric field in thunderclouds is not very large.
Years of experimental measurements from aeroplanes and air balloons have shown that the field is about 10 times smaller than what is needed to initiate lightning.
It is not clear how a lightning bolt is born, but the idea is that something has to “seed” it first.
What do we know about how lightning works?
In 1749 Benjamin Franklin discovered that lightning was an electrical discharge between a thundercloud and Earth.
We know that thunderstorms can generate over 100 million volts of electricity, but we also know that this gets applied across a really large space—hundreds of metres.
So the resulting electric field, or concentration of electric force, is not actually very big.
It is estimated that Earth gets struck by more than 100 lightning bolts every second.
How is that electric current released?
For lightning to propagate from its point of origin to other locations—the ground, for example—the air, which is normally an insulator, must somehow permit electrical charge to move freely.
The lower part of a thundercloud is negatively charged, and as a storm moves, it causes positively charged particles to gather at ground level.
So, as the lightning is triggered, the lower part of the cloud generates a channel of ionized air—or lightning “leader”—that allows electric current to flow freely and transports the negative charge towards positively charged objects, such as trees or buildings. That is when a lightning strike happens. And these currents are huge: They heat the air to about 27,700 degrees Celsius, roughly four times hotter than the surface of the sun.
What are the main theories about what initiates this process?
A hypothesis explored by many scientists is that lightning is initiated when collisions between ice particles in thunderclouds ionize the air.
The ice particles can separate enough electric charge to cause a large electric field and trigger a lightning strike. Another possibility, which I am working on, is what is known as “electron runaway breakdown”.
The idea is that there could be another type of electric discharge, a completely new physical phenomenon.
Your theory is that this other type of electric discharge is caused by cosmic rays. How?
Cosmic rays are high-energy particles, mostly protons, born in and accelerated by energetic astrophysical processes, such as supernovae and star collisions.
These rays travel across space and strike the upper atmosphere of Earth, producing highly energetic showers of ionized particles, accelerated nearly to the speed of light. We can measure these ionized particles with cosmic ray detectors.
Because cosmic rays produce highly energetic showers, for them to trigger lightning in a storm cloud, the cloud’s initial electric field does not have to be very big.
So what do you think happens when cosmic rays encounter a thunderstorm?
Our theory is that when these high-energy particles happen to go through a thundercloud, they ionize the air inside it and create a region with a lot of free electrons, which collide with atoms in the air and produce even more electrons: that is a runaway breakdown. In this case, the initial distribution of electricity in the storm cloud can be across a vast amount of space—several hundred meters or even kilometers. That is because once runaway breakdown is triggered, the cascade of high-energy particles quickly covers large distances. The result is the creation of this very large amount of negatively charged particles, exactly what is supposed to trigger the spark that initiates a lightning bolt. In principle, this is what is happening in thunderclouds, but nobody has yet proved it directly in experiments.
You first introduced this idea two decades ago. What new evidence do you have now?
The results of numerical calculations have demonstrated that this runaway electron breakdown exists. But the theory is always related to some idealized model.
In a real thunderstorm, conditions such as wind and electric field vary in space and time; it is incredibly difficult to prove the hypothesis experimentally. So we decided to take a look at radio pulses that are produced at the inception of a lightning strike. These have been noticed before, coinciding with cosmic rays but never explained.
We wanted to prove that it is the showers of ionized particles caused by cosmic rays penetrating thunderclouds that produce the radio pulses.
How could you demonstrate this?
We used a device that measures radio waves and shows which direction they come from, to record data from 3,800 cloud-to-ground lightning strikes in Russia and Kazakhstan. When we analysed this data we found that, although their pattern matched that predicted by the models of runaway breakdown, the pulses were too big to be produced by regular cosmic rays.
So might there be something amplifying the impact of the cosmic rays?
We suggested an explanation: we know that every storm cloud has tiny charged ice particles called hydrometeors, and we think they might be amplifying the pulses. Our calculations confirm it: when a large amount of free electrons – created in a runaway breakdown process initiated by cosmic rays—collect near these hydrometeors, they boost the current and observed radio pulse signal.
What other experiments are you conducting to prove the runaway breakdown theory?
We are also trying to find a correlation between gamma rays, cosmic rays and radio pulses, because we believe the gamma ray bursts we see above storm clouds are caused by the runaway breakdown process.
Gamma rays are essentially an extremely energetic form of electromagnetic radiation. They are usually invisible, but highly energetic explosions of gamma rays, known as gamma ray bursts, are visible and incredibly bright.
These bursts usually occur in deep space, but they are also observed in Earth’s atmosphere as very bright flashes lasting a fraction of a second above powerful thunderclouds.
How do you observe these bright flashes?
To see gamma radiation you need to be very high up, and our lab at the Tien Shan High-Altitude Scientific Station in Kazakhstan is at almost 4 kilometres above sea level.
We have sensors there that measure gamma radiation; it is very clear that when there are no thunderstorms, there are no gamma ray bursts.
But, in keeping with our theory, as soon as a thunderstorm starts, these bright flashes appear inside the storm clouds – and they correlate with the radio pulses that we register as well.
Will understanding what triggers lightning help us to unlock anything new about Earth and the cosmos?
In my opinion, runaway breakdown could happen in gas, it could happen in space, and that’s what we’re searching for. Analogous processes could take place on the surface of Jupiter, for example. So understanding runaway breakdown here on Earth can help us understand what happens elsewhere in the universe.
What Causes Lightning Bolts?
See also: Dogs and Thunderstorms;
Fast as Lightning
While we don’t understand everything about lightning, here are some things we do know. Lightning is like a giant spark of static electricity.
Particles of ice smash into one another inside of storm clouds, breaking apart and picking up electrostatic charges.
The lighter, positively charged particles gather near the top of the clouds, while negatively charged particles gather near the base.
What happens next depends on lots of complicated factors. One possibility is that once the voltage is high enough, an electrostatic discharge occurs between the two regions of the same cloud, forming what is called intra-cloud lightning.
Sometimes, the discharge can occur between the positively charged region of one cloud, and the negatively charged region of another. This is called inter-cloud lightning.
The kind of lightning most of us are familiar with is called cloud-to-ground lightning. Though a more accurate name would be cloud-meets-the-ground-halfway-lightning, but that isn’t anywhere near as catchy.
In cloud-to-ground lightning, the electrostatic charge occurs between the cloud and the ground.
It’s a rather complicated process, but it starts with electrical “leaders” forking out from the cloud towards the ground in random, jagged steps.
Once these steps get close enough to the ground, electrical “streamers” start to extend from objects on the ground towards the charge coming from the cloud.
Once the leader and streamer connect, a massive electrical charge flows from the cloud into the ground, producing the bright flash called the “return stroke” which we most commonly associate with lightning. The rest of the process is visible as well, but it happens so fast that it’s difficult to see.
One interesting fact is that while the charges flow from the cloud to the ground in the return stroke, the flow starts at the bottom and drains out from bottom to top. If you held a container of water up in the air and poked a hole in the bottom, the water would flow out the same way that electrical charge travels in the return stroke.
Dr. Lee Falin earned a B.S. in Computer Science from the University of Illinois, then went on to obtain a Ph.D. in Genetics, Bioinformatics, and Computational Biology from Virginia Tech.
Why are lightning bolts jagged instead of straight?
William C. Valine, an atmospheric scientist at the University of Arizona, explains.
Ever since Benjamin Franklin's time lightning has been understood to be a large electrical discharge similar to that seen when a conductive object (like a metal doorknob) is touched after a static electric charge is picked up (by feet scuffing across carpet, for example).
But whereas the spark from static electricity measures a centimeter or less in length, a lightning channel can span five kilometers or more. (Also, cloud-to-ground lightning involves electrical currents on the order of tens of thousands of amps. In contrast, a circuit breaker for a common household circuit is usually rated at 20 amps.
) Because of its extreme scale, lightning is a complex physical phenomenon.
During a thunderstorm the lower portion of a cloud contains a region that accumulates a large negative charge and the upper portion becomes positively charged. Also, a positive charge is induced on the surface of the earth, because the negative charge in the cloud is closer to the ground.
Although some lightning strikes, or lightning flashes, transfer the positive charge to ground, such positive flashes are rare in comparison to lightning flashes that transfer negative charge to ground. From the appearance of a lightning flash it is clear that this charge transfer occurs in a channel with a width that is small compared to its length.
Because air is normally an electrical insulator, it must break down so that the conductive channel can form in order for a lightning flash to occur. This breakdown of the air between the cloud and the ground does not happen all at once, however.
Instead, it happens in discrete steps of about 50 meters, with each step taking about one microsecond and about 50 microseconds elapsing between steps. Because of the discrete nature of this process, the initial channel of a lightning flash is called a stepped leader.
Although the formation of a lightning channel is a result of the attraction between negative charge in the cloud and positive charge induced in the ground, the individual steps are often far from vertical.
(Note the small horizontal section of the channel shown in the photograph above.) This variability arises because the conductivity of the air is not uniform.
The channel will tend to extend out to regions of higher conductivity (as shown in the figure by some of the branches actually pointing slightly upward).
As a result, the relatively short step size and the random distribution of such regions of higher conductivity render the channel jagged rather than smooth. Also, the point where the channel contacts the ground may be displaced a considerable distance horizontally from the point inside the cloud where the channel began.
Answer originally published July 7, 2003.